The invention relates to a motor having a rotational drive which can cause an output drive shaft to move rotationally, and having a linear drive which can cause the output drive shaft to move translationally, with the linear drive having a rotor which is caused to move linearly with the aid of a stator and exerts an axial force on the output drive shaft via a motor element which connects the output drive shaft to the rotor.
Motors such as these, in which both a rotational and a linear drive are envisaged, are used for example for injector and metering units in plastic injection machines. In order to allow the injection pressure for the plasticized plastic to be controlled, information is required about the axial force exerted by the linear drive on the output drive shaft.
Until now, the approach has been adopted of determining the axial force on the basis of the stator current which is the cause of it. The motor current has therefore been measured and evaluated. However, the axial force can be determined only inaccurately, and in particular with errors, using this method. The determination process is inadequate when high axial force measurement accuracy is required.